IONIC-CONDUCTIVITY ENHANCEMENT OF THE FLUORIDE CONDUCTOR CAF2 BY GRAIN-BOUNDARY ACTIVATION USING LEWIS-ACIDS

Following the concept of ''heterogeneous doping,'' CaF2 fine powder and films were treated with the Lewis acids SbF5 and BF3 to create a strong driving force to draw F- ions out of the boundary regions according to SbF5(surface)+ F-F(boundary) --> SbF6-(surface) + V-F(.)(boundary) The powder has been vacuum-treated with SbF5 and BF3 vapor and then processed to pellets. The conductivity results indicate a significant enhancement of the ionic conductivity with a slope that is characteristic for F--vacancy migration. A simple homogeneous doping by Sb and B impurities would lead to a conductivity via interstitials and thus to a different activation energy. The enhancement should not be due to oxygen impurities introduced by the heat-treatment: this is shown by preparation in different gas atmospheres. A quantitative estimation of the enhanced conductivity assuming space-charge effects is presented. Conductivity results on thin films also confirmed the assumption of space-charge regions as a consequence of the attractive forces of the adsorbed Lewis acid.